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Just a few days prior to its monthly patch release, Microsoft released an emergency patch for a critical vulnerability in the Windows Host Compute Service Shim (hcsshim) library that could allow remote attackers to run malicious code on Windows computers.

Windows Host Compute Service Shim (hcsshim) is an open source library that helps "Docker for Windows" execute Windows Server containers using a low-level container management API in Hyper-V.

Discovered by Swiss developer and security researcher Michael Hanselmann, the critical vulnerability (tracked as CVE-2018-8115) is the result of the failure of the hcsshim library to properly validate input when importing a Docker container image.

This, in turn, allows an attacker to remotely execute arbitrary code on the Windows host operating system, eventually letting the attacker create, remove, and replace files on the target host.

As Hanselmann explained in his personal blog, "Importing a Docker container image or pulling one from a remote registry isn't commonly expected to make modifications to the host file system outside of the Docker-internal data structures."

Hanselmann reported the issue to Microsoft in February this year, and the tech giant fixed the vulnerability a few days before this month’s patch Tuesday by releasing an updated version of hcsshim.

Although the vulnerability has been assigned a critical severity rating, Microsoft says exploitation of this issue is unlikely.

"To exploit the vulnerability, an attacker would place malicious code in a specially crafted container image which, if an authenticated administrator imported (pulled), could cause a container management service utilizing the Host Compute Service Shim library to execute malicious code on the Windows host," Microsoft says in its advisory.

The patch for this vulnerability addresses the way hcsshim validates input from Docker container images, therefore blocking the loading of malicious code in specially crafted files.

An updated version 0.6.10 of the Windows Host Compute Service Shim (hcsshim) file is available right now for download from GitHub.

Full details of the vulnerability have not been released yet, but Hanselmann promises to publish in-depth technical details and a proof-of-concept exploit for the flaw on May 9, following an agreement with Microsoft security response center.

Microsoft's May 2018 Patch Tuesday has been scheduled for release on May 8.